In recent years, there has been active research conducted relating to TFTs (thin film transistors) obtained by using organic materials or amorphous oxides. Thin film transistors are transistors comprised of a substrate on which a semiconductor layer, gate electrode, source electrode, drain electrode, and gate insulating film are formed and are roughly divided into bottom gate types and top gate types. A bottom gate type is one which has a gate electrode on the substrate and has a source electrode and drain electrode connected by a semiconductor layer through a gate insulating film. On the other hand, a top gate type is one which has a source electrode and drain electrode adjoining a semiconductor layer on the substrate and has a gate electrode on that through the gate insulating film.
The gate insulating film forming part of such a thin film transistor is required to be excellent in insulating ability and electrical characteristics. A high dielectric constant gate insulating film enables a transistor to be driven at a low voltage and contributes to lower power consumption of the device. Further, for example, in the bottom gate type, when forming a semiconductor layer and a gate electrode and source electrode in predetermined shapes on the insulating film, for example, the photolithography method and the etching method are sometimes used. In this case, the gate insulating film is contacted by the developing solution and the etching solution, so the insulating film is required to have chemical resistance. In addition, such a formed gate insulating film is later formed with a semiconductor layer, gate electrode, and source electrode over it, but when forming the semiconductor layer, gate electrode, and source electrode, high temperature treatment is performed, so the gate insulating film is required to hold a high adhesion even after such high temperature treatment.
As the resin for forming such a gate insulating film, for example, Patent Document 1 discloses the art of using polyvinyl phenol (PVP) to form the gate insulating film. However, if using polyvinyl phenol like in this Patent Document 1, while it is possible to obtain a gate insulating film which is excellent in chemical resistance to a certain extent, there is the problem that the obtained gate insulating film is remarkably low in dielectric constant. Further, Non-Patent Document 1 discloses the art of forming a gate insulating film which is comprised of cyanoethyl pullulan and a cross-linking agent having a triazine ring, but such a gate insulating film remarkably falls in adhesion with the substrate if exposed to a high temperature after forming a film.